A method and apparatus for solid phase welding of pipes is disclosed in U.S. Pat. No. Re. 28,190, dated Oct. 8, 1974, in which I am co-inventor. A study of such patent, which is particularly pertinent as exemplifying relevant prior art, discloses a method and apparatus for joining adjacent ends of pipes together by means of solid phase or pressure welding.
In more detail, such a system utilizes heating the adjacent pipe ends and physically urging the pipe ends towards each other to create an upset pressure at the pipe ends. An suggested in said patent, the mechanism for effecting the axial translation of the respective pipes includes a plurality of internal pressure pads which are disposed internally at each pipe end, and which are also moveable radially against the inner surface of the pipe so as to forcibly clamp the pipe. Such a mechanism was actually intended to serve three functions; the first being to bring each pipe end into circular shape; the second being to place the two pipe ends in axial alignment; and the third being to act as the vehicle for axially urging the pipe ends together.
Subsequent study and experimentation has indicated that the first function listed above is best served if the pads are relatively short, as measured along the axis of the pipe, while the third function is enhanced by increasing the axial extent of the pads, so as to provide a better frictional grip as the extremely large upset pressure is applied.